Semiconductor based solar cells pass solar radiation from a front side of the solar cell through one or more photoactive regions to a back side of the solar cell. Charge carriers are generated due to absorption of photons in the photoactive region. Each photoactive region may be associated with one p-n junction. For some devices, a given photoactive region absorbs light over only a limited range of wavelengths, depending on the materials used to form the p-n junction. In order to increase the solar cell's efficiency, the p-n junctions may be formed from different materials such that light over a greater range of wavelengths can be absorbed. This is typically referred to as a “multi-junction” design.
However, the charge carriers that are generated by each photoactive regions need a path to flow. In some devices, “tunnel junctions” allow charge carriers to flow through the solar cell. For example, a separate tunnel junction may join each part of the solar cell that has a different photoactive region. Unfortunately, tunnel junctions can be difficult to fabricate.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.